This invention relates to a flywheel driveline and control arrangement, suitable for use as part of an energy storage and recovery system such as may be incorporated in a hybrid vehicle.
In a high-speed flywheel-based energy storage and recovery system, the flywheel is connected to the transmission of the vehicle via a continuously variable transmission (CVT) and manipulation of the CVT ratio achieves control of energy storage and recovery. See, for example, SAE technical paper 2008-01-0083, Apr. 14-17, 2008.
When the ratio is set so as to speed up the flywheel, energy is stored and when the ratio is set so as to slow down the flywheel, energy is recovered.
The key advantage of a flywheel hybrid vehicle is the magnitude of the power that can be transmitted between the flywheel and the vehicle wheels. A road car is capable of very high power transfer during braking and the key to hybrid system effectiveness is capturing as much of this normally wasted energy as possible. This is achieved by the careful matching of the flywheel rotational speed to that of the vehicle. The rotational speed of a high-speed flywheel varies approximately between zero and 60,000 RPM, whereas the vehicle's speed varies from zero to over 160 kph. There are two extremes of real-world operating conditions. The first is when the flywheel has zero speed and when the vehicle's speed is maximum. The second is when the flywheel is at its maximum speed and when the vehicle's speed is zero. To match these extreme conditions, ideally an infinitely variable transmission is required, or alternatively a CVT transmission used in conjunction with at least one clutch. The second option is perhaps more pragmatic as CVT's are a known and mass-produced transmission. Unfortunately, a CVT has a finite ratio range of typically six. This ratio range is inadequate to cover the entire operating spectrum of flywheel and vehicle speed, even with appropriately selected interim gear ratio sets. Consequently, the interim gear sets are usually optimized for a given goal such as maximizing fuel consumption savings on a defined drive cycle. However, this is likely to compromise fuel consumption savings at other operating conditions, especially those outside the boundaries of the defined drive cycle.
As described in Applicant's co-pending Application GB0816109.3, selectable multiple gear sets can be incorporated within the transmission between the flywheel and the vehicle wheels. The inclusion of one or more additional selectable gear sets beneficially broadens the operating window of the energy storage and recovery system. However, even with these additional multiple gear sets it is not possible to transfer power between the flywheel and the vehicle's wheels under the extremes of operating conditions. This limits the potential for fuel consumption savings. As it is not possible for the CVT transmission system to always match the vehicle and flywheel speeds, a clutch, interposed between the CVT and the flywheel is generally deployed to dynamically bridge the speed difference.
In conditions when the flywheel is at maximum speed and the vehicle is close to stationary, a substantial amount of energy (˜400 kJ) needs to be dissipated through the clutch. At the other extreme condition, when the flywheel is at zero speed and the vehicle speed is close to maximum, a considerable but lesser amount of energy (˜100 kJ) needs to be dissipated through the clutch to bridge the speed difference. These conditions represent wastage of energy and can lead to premature wear of the clutch itself.
This invention aims to mitigate the aforementioned problems.